An Assessment of Immersion Silver Surface Finish for Lead-Free Electronics

Abstract

The worldwide transition to lead-free electronics has increased the usage of several lead-free pad finishes for electronic assembly manufacturers, including immersion silver, immersion tin, electroless nickel-immersion gold, and organic solderability preservatives. This study assesses and compares immersion silver as a circuit board finish in terms of its ease of use, wettability, solderability, shelf life, appearance, solder joint strength, intermetallic and void formation, reliability, and costs.

Electroless and Immersion Plating Process towards Structures and IMC Formation

“…Immersion silver (ImAg) is one of an alternative finish where it was designed as protective surface finish to ensure the solderability of the underlying substrate [10,15]. The advantages of ImAg finish including good coplanarity, wire bondable [15], and suitable for fine-pitch of electronic applications [67].…”

“…During soldering process, ImAg will be dissolved into the molten solder during assembly process because of liquidation of silver into the tin (Sn) based in the solder [10,15] as shown in Fig 7 (a). The thickness of Ag layer is typically less than 1 µm.…”

“…Furthermore, Cullen [68] stated that the ImAg consist of 0.15-0.55µm thickness layer and it is 100 times thinner than the traditional electroplated silver deposits. In addition, Wang et al [10] found that in order to get the 0.5 µm Ag layer in range [69], the duration of plating time is around 1 minute to 4 minutes. The Association Connecting Electronics Industrial standard (IPC-4553) [70] stated that the recommended thickness of Ag layer as minimum 0.13µm and typically 0.2µm-0.3µm can be obtained by using 60 x 60 mil pad.…”

“…Immersion silver (ImAg) is one of important surface finish for the electronic assembly and it was Restriction of Hazardous Substances (RoHS) compliant. ImAg also known as an alternative to HASL [10][11][12] and ENIG [13,14] because of their good solderability, wettability, co-planarity and bonding performance [10,15]. According to Wang et al [10], the selection of immersion silver as a final surface finish is a good decision because it can eliminates the embrittlement of Au-Sn intermetallic layer (IMCs) as well as provides simpler operation and lower cost.…”

“…ImAg also known as an alternative to HASL [10][11][12] and ENIG [13,14] because of their good solderability, wettability, co-planarity and bonding performance [10,15]. According to Wang et al [10], the selection of immersion silver as a final surface finish is a good decision because it can eliminates the embrittlement of Au-Sn intermetallic layer (IMCs) as well as provides simpler operation and lower cost. Furthermore, immersion plating are not required to use reducing agent because the base material can behaves as reducing agent compared to electroless plating process [16].…”